1. Field of the Invention
The present invention relates to a fluid-filled vibration damping device that uses a vibration damping effect based on the flow action of non-compressible fluid filled inside.
2. Description of the Related Art
As one type of vibration damping device, such as a vibration damping connecting body, a vibration damping support body or the like, interposed between members constituting a vibration transmission system, there is a vibration damping device including a first mounting member, a second mounting member, and a main rubber elastic body elastically connecting the first mounting member and the second mounting member. A fluid-filled vibration damping device has been known as an evolved model of the vibration damping device. This fluid-filled vibration damping device includes a pressure receiving chamber whose wall is partially constituted by the main rubber elastic body and an equilibrium chamber whose wall is partially constituted by a flexible film. Non-compressible fluid is sealed in both chambers, and both chambers are in communication with each other through an orifice path. This kind of constitution makes it possible to exhibit a vibration damping effect owing to flow action, e.g., the resonance action of fluid that flows through an orifice path due to the difference in pressure between the pressure receiving chamber and the equilibrium chamber upon input of the vibration to the pressure receiving chamber. The fluid-filled vibration damping has been applied, for example as an automobile engine mount, a body mount, a differential mount, a suspension member mount, a suspension bush or the like.
The engine mount of an automobile or the like is requested to exhibit respective vibration damping effect for a plurality of frequency ranges. To meet this end, the orifice passage is tuned to a low frequency large amplitude vibration, such as an engine shake or the like, and a movable membrane is further provided for absorbing the pressure fluctuation of the pressure receiving chamber at a high frequency small amplitude vibration such as booming noise or the like.
In addition, with automobile engine mounts and the like, a problem of vibration and noise occurring during input of excessive vibration load or impact load have been overrated. This is thought to mainly be caused by cavitation bubbles that accompany the generation of an excessive negative pressure in the pressure receiving chamber. Specifically, when the pressure receiving chamber to which a large amplitude vibration has been input goes to an excessively negative pressure state, the air dissolved in the fluid of the pressure receiving chamber undergoes liquid phase separation, and cavitation bubbles are formed. Then, it is thought that the water hammer pressure that accompanies the collapse of the bubbles is spread to the first mounting member and second mounting member, and by being transmitted to members constituting a vibration transmission system such as of an automobile body or the like, problematic noise and vibration are generated.
To deal with this problem, the present applicant proposed previously in U.S. Pat. No. 8,556,239 a novel structure for which a connecting hole is provided in a partition member for partitioning the pressure receiving chamber and the equilibrium chamber in order to have both chambers communicate, and a closing rubber elastic plate for closing the connecting hole is overlapped from the pressure receiving chamber side on the connecting hole to constitute a connecting hole communication and blocking control means. When a rapid pressure decrease occurs in the pressure receiving chamber upon input of an excessive vibration load or impact load, the connecting hole goes to a communication state by the closing rubber elastic plate being elastically deformed and separating from the partition member, and the pressure receiving chamber and the equilibrium chamber become short circuited, and thus negative pressure being generated in the pressure receiving chamber is avoided. Also, this closing rubber elastic plate, by exhibiting an absorption function of the pressure fluctuation of the pressure receiving chamber by being elastically deformed in a blocked state, exhibits a vibration damping effect on high frequency, small amplitude vibration.
The present inventors made further investigation of the fluid-filled vibration damping device of U.S. Pat. No. 8,556,239 and came to believe that there is still room for improvement. Specifically, with the communication and blocking control means noted in U.S. Pat. No. 8,556,239, an elastic deformation area of the closing rubber elastic plate for which the connection hole is opened by elastic deformation is provided between both circumference ends of an abutting retaining part, making it difficult for the elastic deformation area to separate from the opening of the connecting hole, during input of normal vibrations such as an engine shake, a driving rumble or the like. Because of that, the vibration insulating action on the input of high frequency small amplitude vibration is exhibited by the fluid pressure absorption function through elastic deformation of the closing rubber elastic plate covering the connecting hole. This may possibly cause a certain amount of limitation of the vibration insulating action due to the deformation rigidity of the closing rubber elastic plate.
To advantageously obtain the vibration insulating action, it is conceivable to make the deformation rigidity of the closing rubber elastic plate markedly smaller. However, this is likely to cause more readily elastic deformation of the closing rubber elastic plate based on the pressure difference applied to both the front and back surfaces of the closing rubber elastic plate, during input of vibration such as the engine shake or the like. As a result, the relative pressure difference between the pressure receiving chamber and the equilibrium chamber becomes smaller, the flow action volume of fluid passing through the orifice passage decreases, leading to difficulty in sufficiently exhibiting the vibration damping effect by the orifice passage against the low frequency large amplitude vibration.